A workpiece may define many protection holes on a periphery. During machining, a clamping device may be employed to protect the protection holes, thereby avoiding damage to the protection holes. The clamping device includes a supporting seat, a first clamping assembly, a second clamping assembly, and a pressing plate. When in use, the workpiece is sleeved on the supporting seat, the first clamping assembly and the second clamping assembly is movably mounted on the supporting seat and tapped into corresponding protection holes. Then, the pressing plate is securely mounted on the supporting seat and presses the first clamping assembly and the second clamping assembly upon the supporting seat. When disassembling the clamping device from the workpiece, operators may disassemble the pressing plate from the supporting seat, and pull the first clamping assembly and the second clamping assembly from the protection holes. However, the pressing plate, the first clamping assembly and the second clamping assembly are disassembled from the supporting seat manually, which results in labor-consuming, complicated operations, and low efficiency.